The aim of the proposed research is to isolate and characterize H-2 antigen variants. The variants will be isolated from heterozygous mouse leukemia cells, using two immuno-selective agents: (1) anti H-2 antisera and complement and (2) cytolytic T-cells. Survivors of the selection will be characterized by a combination of biochemical, genetic and immunological techniques. Using this approach, we intent to achieve the following goals: 1. In the past few years, we have isolated several variants using antisera. The genetic mechanism involved is not clear, but mitotic recombination resulting in homozygosity for the expressed antigens is a possibility. Experiments designed to test this possibility are described. 2. We will attempt to determine the nature of the T-cell target antigen by establishing a selection system to obtain resistant variants using cytolytic T-cells. By studying the nature of the defect in the cells resistant to lysis, we hope to be able to gain insights into the identify of the target antigens(s) of T-cells. 3. Using an anti H-2K kappa hybridoma as the immunoselective agent, we have isolated what appears to be a structural mutant. We will attempt to define the nature of the mutation in this variant. Other variants will be isolated using the same and other hybridomas against H-2K kappa to determine the nature of antibody reactive sites on the H-2 mutants.